how can i buy more storage

Upgrade your iCloud storage: https://support.apple.com/en-us/HT201318

If you are asking about buying more storage for your phone, you can't. As has already been provided, if you need to sync more data with the cloud, you can buy more space in the cloud, but that will NOT increase the memory on your phone. iPhone memory is fixed at purchase and can never be increased.

I what to buy More storage